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As traffic heads to the beach, emergency crews build a temporary dam protecting U.S. 501 from floodwaters after Florence struck the Carolinas, Monday, Sept. 17, 2018, in Conway, S.C.

Two years ago, from Hurricane Matthew and in 1999, from Hurricane Floyd, floodwaters came close to entering Joe Holmes' house in South Carolina, but he dodged those bullets. Now, with Florence , he doesn't feel so lucky.

He worries the Waccamaw River will make its way into his Conway home because the state wants to save the main highway into Myrtle Beach, a more densely populated city and tourist destination, from going underwater.

Officials insist they must keep the road open, or hundreds of thousands of people would be isolated. So the state is building a higher wall — in the form of a second level of concrete highway barriers on top of the sides of 1.5 miles (2.5 kilometres) of bridges and causeways, sealing them with plastic sheets weighed down with sandbags and rebar — in its last stand on U.S. Highway 501.

Skeptical Conway residents fear this will push water into their neighbourhoods, but officials say it won't affect any areas that weren't already doomed to flood — at least not in the models they've run.

"It's never been done before. They can't say for certain what it might do," Holmes said Monday, surveying flooded areas in his golf cart. "Why is that highway so important? Why can't they just fly food and fuel in? We've got a big airport."

The Waccamaw River — which winds through Conway, population 23,000 and home to Coastal Carolina University — was already in a major flood at just over 15 feet (4.5 metres) Monday, according to the National Weather Service. The city had received 16 inches (41 centimetres) of rain from Florence's slow march inland, and homes were already threatened. By Friday afternoon, the river should top the record of 17.9 feet (5.5 metres), set just two years ago in flooding from Hurricane Matthew, which was just inches higher than the river's crest in Hurricane Floyd in 1999.